How to overcome non uniform thickness of coating in electroplating

Electroplating usually coats a non-uniform thickness film. I would like to know how this can be overcome such as in high precision electroplating. Thanks in advance.

Regards,

Robin Francois- Boston, Massachusetts

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When electroplating things, they are immersed in an aqueous (water-based) solution, and a source of direct current is attached; in the industry we often call that power supply device a 'rectifier'. The piece you are plating onto is negatively charged (made the cathode), and the positively charged electrode is called the anode. Sometimes the anode is designed to dissolve into the solution to be the source of the plated metal. For example, if you are plating something with nickel, the anode will be made of solid nickel, or nickel chunks will be placed into a perforated titanium basket.
As electricity flows through the external circuit, electrons are pulled from the anode and flow through the external circuit to the cathode. The anode now has a deficit of electrons, meaning that a proportionate amount of the anode nickel will be converted from nickel metal (Ni0 atoms) to Ni++ ions. The nickel anode metal is thus dissolving into the solution as positively charged nickel ions.

Meanwhile, the cathode is accumulating a surfeit of electrons, and this is attracting those positively charged nickel ions to it. When they reach it, the electrons neutralize the positive charge on the Ni++ ions, and they become nickel metal (Ni0 atoms) once again. It takes two electrons to reduce one Ni++ ion to an atom of nickel metal (Ni0). Thus there is a direct correspondence between how much electricity flows and how much metal is deposited. In fact, "Faraday's Law" says essentially that 96,487 ampere-seconds will deposit one gram equivalent weight of the metal. An ampere-second is one ampere flowing for one second; if the product of the amperes and seconds is 96,487, that deposits one gram equivalent weight of metal.

But imagine that the current is off, and you instantaneously go to full throttle. The electrons move at the speed of light from the anode to the cathode, but the dissolution of the anode and migration of the ions though the solution happens much slower. So what happens at the cathode when there is a huge excess of electrons accumulating, and insufficient nickel ions to satisfy them? They start pulling hydrogen (H+) out of the water, leaving hydroxide (OH -) behind. As the H+ meets the electrons, the hydrogen bubbles out of the solution as hydrogen gas. So Faraday's Law actually tells us that 96,487 causes one gram equivalent weight of something to be reduced, but it might actually work out as 90% of a gram equivalent weight of nickel and 10% of a gram equivalent weight of hydrogen. This would be called 90% efficiency. Too much current density will cause too much of this inefficiency and other difficulties which result in a roughness and poor plating defect called 'burning'.

Ignoring this 'efficiency' stuff for a bit, we see that the amount of plating is directly proportional to the amount of current. Thus, if a component has some spots which are close to anodes or for some other reason draw more current than other areas, the plating will be thicker there. This thread is basically about what can be done about that situation ...

The easiest fix, but only partially effective, is organic addition agents (brighteners) added to the solution which are also drawn to those high current density areas and get in the way of metal deposition, thus assisting in making it more uniform.

Other things that can be done besides ideally locating the anodes with respect to the parts include:
1. Thieves -- These are metal pieces connected to the cathode, designed to draw electricity themselves, and placed very near the high current density area of the part so they steal some of the electricity and prevent the actual part from 'burning'. Thieves represent deliberate waste and are rarely used except in chrome plating.
2. Shields -- These are pieces of non-conductive plastic of any size or shape, usually made of PVC or polypropylene, which are placed in specific spots in between the anode and cathode to block the 'shortest path' to minimize burning and achieve a more uniform current and thickness distribution.
3. Auxiliary Anodes -- These are metal pieces, sometimes made of plating material, but more often made of non-dissolving metals, which are connected to the anode and placed in specific places near the cathode. For example, if you wish to fully plate a wheel or a truck bumper which has deeply recessed areas, you might put an auxiliary anode near that area to reduce the anode-cathode distance so more current will flow to that area.

So if the current distribution is uniform, the plating thickness will be uniform. But to even out the thickness when geometry favors higher or lower current density to certain areas, three "mechanical" techniques are shielding (the use of plastic shields to block off the shortest path), thieving (the use of conductive wires as cathodes to steal some of the current away from high current areas, and auxiliary anodes (anode material in close proximity to the work in the areas which need more current).

One or more of these should fix your problem. Good luck.

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

(2005)

Q. What are the wetting and leveling agents that are used in Microfabrication?

In MEMS fabrication, I have heard that they use wetting agents and leveling agents in order to be able to electroplate in very small areas such as deep holes for example. What are those agents? Are there other agents or other ways that can be used to do that? And what is the effect of those agents on the plating process?

Robin Francois [returning]- Boston, Massachusetts

(1999)

A. Hello again Robin.

"Brighteners" are organic addition agents that are drawn to high current density areas and, by being there, tend to somewhat shield those areas from plating, thus driving the plating towards occurring elsewhere. They are often used in plating of all types to force more adatoms for a denser, harder, brighter, less porous plate. To an extent they may also contribute to more uniform plating thickness.
In the case of MEMS and thru-hole plating in circuit boards, the brighteners are such large molecules that they cannot easily fit into the holes; thus they shield everything but the holes they can't fit into, and that promotes the leveling. These agents are proprietary; the suppliers who have spent small fortunes developing them and will not tell you what they are; rather, they will simply sell you the copper plating bath or brightener so you effectively license the technology "by the gallon".

For your MEMS issues, L.T. Romankiw's "A path: electroplating through lithographic masks in electronics to LIGA in MEMS" published in Electrochimica Acta, Vol 42, Nos 20-22, pp. 2985---- by Elsevier Science is invaluable. I have what looks to be a reprint from Pergamon Press labeled as "PII: S0013-4686(97)00146-1", whatever that means. Good luck finding one. If neither Elsevier not Pergamon has it, you might contact IBM Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598 since they sponsored much of the work.

Good luck.

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

(1999)

A. If the object to be plated is not mechanically complex, you can try plating using anodes of the same shape in close proximity. Thieving, additional anodes, and shielding are otherwise essential for complex shapes.

Jeff Verive- Bolingbrook, Illinois

A. Hi Robin. All plating is not necessarily Electroplating. You can achieve very even finishes all over parts by using autocatalytic plating (ELECTROLESS Plating). There are Electroless solutions available that will provide answers to most finishing situations, but the cost is not cheap.

Regards

John Tenison - Woods- Victoria Australia

November 27, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear Sir,

Can you suggest some automation methods to form uniform coating thickness .
We carry out copper plating in very conventional manner and the deviation of the coating thickness varies a lot, up to 100-200 microns sometimes.

Is the any sort of automation process that can help to control the variation in microns?

Or should I find a rectifier of higher pulsing rate or anything else?

Please help.

Jack RomanEngineer - Mumbai, Maharashtra, India

November 28, 2012

A. Hi Jack. We appended your question to an existing thread which may go a long way towards answering it. Properly designed plastic shields may be the easiest resolution, but send us a pic or sketch of the parts and the racking and we may be able to refine the answer. Good luck.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

December 26, 2012

Q. I have recently developed a plating rack that holds 198 lug nuts/Wheel locks. I modified the post length (current rack has all the same post lengths) to change the distance between the anode and the work. The result is almost completely uniform plating within 0.1 mils for nickel. The rack looks similar to a lens or a 3D diamond shape now. We reduced the variation in thickness from around 16-20% across the rack to less than 2%. That 2% is coming from the 8 corner pieces now. Without using conductive thieves (wasting nickel) how can we stop the build up in the corners? Any ideas? I can send a CAD drawing of the current rack and the new "modified rack" to an email.
I really appreciate any help!!
Sincerely,

Thomas Dobmeier Jr.- Buffalo, New York, USA

December 27, 2012

A. Hi Thomas. Shields are the easy way. These are simply small PVC or polypropylene strips through which current cannot pass, but must go around. They thereby increase the anode to cathode distance to whichever lug nuts they are shielding. They do not need to be solid sheets either: they can be perforated to only partially block the path from anode to cathode, thereby reducing the current to that area.

Unless you want to invest in electrochemical modeling software, though, it will take a lot of tinkering and trial-and-error to get the exact results you seek.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

Improving throwing power on nickel baths

May 29, 2014

Q. Hello all.
I need to improve the nickel throwing power in parts that we currently plate for a customer in US.
Those parts are ABS,so they enter copper bath first, having good covering on very recessive area. The problem is that when they go into the semi bright, then bright nickel this recessive area is not 100% covered, getting a defect we call "exposed copper".
I have tried painting this area using a chrome spray paint, but our customer does not accept it.
Some plating conditions:
- semi bright: 11 minutes, 4 amp/dm, ph:3.8, 60 °C
- bright nickel: 11 minutes, 4.5 amp/dm, ph:4, 60 °C.
Thank you in advance for the help.
Bernardo

Bernardo Roqueprocess engineer - Guadalajara, Jalisco, Mexico

June 2014

A. Hi Bernardo. I doubt that you will achieve the results you need by a chemical adjustment to your nickel plating baths. I think you should look into auxiliary anodes as the most promising solution. Best of luck.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

June 11, 2014

A. Hello Bernardo,
Ted gave you good advice regarding the Aux. anodes. Another thing you can try is to increase the solution agitation and employ (if not already doing) cathode rocking movement toward the anodes. A slightly higher bath temp will also help. Check your Data Sheet on the bath to operate at the higher end of the range. I don't know what your parts look like, so I hope this helps you.

Mark BakerProcess Engineer - Malone, New York USA

June 11, 2014

A. Bernardo,
If poor covering power is a persistent issue you could concentrate on current distribution, if it is recent or chance issue then look at contamination like copper or any metals.

Venkat Raja- Kitchener, Ontario, Canada

December 1, 2014

Q. I'm having a problem getting uniformity in electroplating Nickel-cobalt on a 12X12 square glass substrate with a photo resisted mask.(33 gal tank with sparger at the bottom running through the center of the anode and cathode). No matter what I do I get Bottom heavy. I've moved the sparger toward the anode; it caused my plating to be slower but still bottom heavy. When I moved it toward the cathode it plated faster but still bottom heavy. Same thing when I had the sparger straight vertical always bottom heavy. I've tried shielding, flipping it, plating very slow -- still same results. Do you think it's how far the sparger is replenishing solution at the bottom causing the bottom to plate quicker? Have so many question no one to give me answers.

hero chea - lowell, Massachusetts

December 2014

A. Hi hero. No, I don't think so. Although everything matters to some extent, I don't think the position of the sparger is the main driver. The main problem is that the current density on the bottom of the rack is too high. For one thing, your anodes need to be shorter than the length of the rack, i.e., the lowest point of the work in the tank must be lower than the lowest point on the anodes. For another thing, it is always possible to build shields which will prevent heavy buildup. Good luck.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

December 2, 2014

A. Hi Hero

12x12 what? mm, feet yards? And a glass substrate is not noted for its conductivity!

However to address your problem try this.

Put an anode horizontally on the bottom of the tank and smaller than the workpiece. (insulate the lead or that will act as a side anode)

Turn the sparger off and suspend the work piece horizontally above the anode and FACE UP away from the anode. It will need to be some way below the bath surface to allow a reasonable current path. It may still be worth using some shielding for the edges but it should help.

Geoff Smith Hampshire,
England

December 2, 2014

A. I agree with Ted. What many people do not consider is some of the + charged ions go in all directions from the anode. Some reflect off of the bottom of the tank, thus heavier plate on the bottom of the part.
Shields and robbers work but it is simpler to just raise the anode up if you do not have shorter anodes.
I have heard of people taping off the bottom portion of the anode. I am not in love with this idea, but it is a quick and dirty check of how short you need the final anode.

James Watts- Navarre, Florida

December 15, 2014

Q. Should I build a shield for the work piece or a shield for the anode? If these pictures of my setup will help...

That's the tank, work piece and anode basket but the basket I'm using right now is smaller then that one.

hero chea [returning] - lowell, Massachusetts

December 2014

A. Hello again Hero. Are you bringing the power into your rack via an alligator clip to a small diameter titanium screw? Not very reliable.

I can't totally figure out your racking (and shielding, if any). And I guess I'm seeing that your part is glass and we can see through it, but I'm not sure. This brings up the point that Geoff implied, regarding how is the glass metallized so you can electroplate it.

But the point that James and I were making is that it looks like your anode basket may be too deep. It should end 2 or 3" further from the bottom of the tank than your plated part ends. If the only problem is a bottom-heavy deposit, try that. For a quick trial, just wrap plater's tape completely around the several bottom inches of the basket as a shield, so that no current can flow from the bottom several inches. Good luck.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

December 15, 2014

A. Hello Hero,
I had to make adjustments to Acid Cu anode baskets in the PC board industry. We were getting 15% higher thickness variation on the bottom of the board. The round Ti anode baskets came 2" below the work. This was an error with the original design / installation of the tanks. Instead of buying 48 shorter anode baskets, we cut 5" lengths of PVDF pipe and slid them onto the outside bottom of the basket. The piping fit snug over the baskets, then we bagged them. We were using 2" high phos cu balls. This solved the thickness variation problem. Because you are using rectangular baskets you could have a plastic fabricator in your area make you rectangular pieces of PDVF to slide onto or fit inside the bottom of the anode baskets. Ted had a good idea of using plater's tape around the bottom of the basket as a test before you take further measures. This anode basket adjustment would save you time in rigging a cathode shield every time you plate your part if a permanent shield on the rack is not possible. Hope this helps!

Mark BakerProcess Engineer - Phoenix, Arizona USA

December 19, 2014

Thanks it worked!! What do you recommend using instead of the alligator clips for power?

Hero chea [returning] - Lowell, Massachusetts

December 2014

A. Hi again. I think the alligator clips are okay, but that they should be clipped onto something more substantial so the current doesn't have to travel through that small screw to get to the work.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

December 30, 2014

Q. I'm trying to get thickness at +/-.00005. Picture of the mandrel is in my last posts. Right now I'm getting .00165 to .00240 throughout the whole mandrel. Any suggestion on how to go about this?

hero chea [returning] - lowell, Massachusetts

December 2014

A. Hello again Hero. Your present variation is +/- .000375, and you want to reduce it to .00005 ... that is a huge reduction in tolerance, and I'm not sure whether you can get it even with endless trial & error on shield designs. The first thing I'd do is to plate two substrates as identically as you can, and measure the thicknesses on both the same way to be certain that any thickness variation is reliably tracking the "geography" of the substrate, and is not being influenced by factors that are still "random".

If you are at the point, or can get to the point, where there is essentially no thickness variation other than that which is clearly mappable to the location on the substrate, then you can start blocking the current to the HCD areas with shields. Good luck.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

December 30, 2014

A. You might want to try increasing air agitation and using a lower amperage, but not so low as it becomes a solution dummying operation.

James Watts- Navarre, Florida

January 1, 2015

A. Hi again Hero

You appear to be using very basic technology to attempt to push the boundaries of this technology.
The surface area of your anode is vast compared with the cathode. The effective area of a basket full of pieces is much more than the superficial area. A single solid anode would be much easier to control.
A 2" square anode at least 6" from the centre of the cathode would be a good place to start. Possibly more but certainly not less.
Alternatively a 2" square hole in a plastic sheet in front of the anode basket. The sheet should be considerably larger than the basket and preferably reach the sides/bottom of the tank.
Turn off the sparger during plating to avoid flow effects
The method I originally suggested has been used to make thickness standards and worked well.
It is not clear from the photo what part of the workpiece is to be plated -- the whole area minus etched lines or just the small rectangles.
How are you measuring thickness? Is the instrument calibrated and what is its basic precision?
Similarly, how good is your rectifier? A calibrated ammeter is essential and preferably a constant current source.
You do not state units or a leading zero so I assume you are getting 0.00165 inch to 0.00240 inch. I make this 42-61 microns which avoids mis-counting all those zeros.
You do not give any idea what the final application is. It could be that electroplating is not the best technique. If electroless nickel could do the job the problem would be much simpler.
I agree with Ted. Until you have established that your current method is reproducible it is pointless trying to improve it.
If you would like to map the current flow in the solution you may like to try Finite Element Analysis but this is advanced theoretical stuff.

P.S. I love the concept of ions reflecting from the bottom of the tank. This is not how ions work

Geoff SmithHampshire,
England

December 2014

Thanks Geoff. Okay, 'reflecting' was not the precise word, but it may help people recognize & visualize that the current flow & ion movement is not restricted to the shortest path, but includes bulged out paths from anode to cathode that may reach the bottom of the tank. Maybe the magnetic lines of flux around a horseshoe magnet are a good visual of the ionic current flow from anode to cathode?

I discovered the implications of this when I assumed that 18 volt rectifiers would be plenty big for a particular nickel plating installation because they were 50% higher voltage than hundreds of successful installations before it :-)
... but in this case the anode-to-cathode distance was large AND we were plating very large solid sheets/plates where there was essentially no bulged out paths, and "gut feel" betrayed us. In this case although the solution resistivity was where it was supposed to be, the solution resistance was very much higher than we expected because there are no bulged out paths when you're plating a huge solid sheet.

There are a couple of companies that offer "electrochemical modeling software" that incorporates Finite Element Analysis, and they can probably be found by googling.

Regards,

Ted Mooney, P.E. RETfinishing.comPine Beach, New Jersey

November 18, 2015

Q. Hi Ted and others

I am very much interested in shielding, Ted mentioned it as a suggestion to a previous posting of mine concerning issues of high field strength around the edge of a hemispherical bowl (acid copper plating the interior).

I have been struggling a little at the conceptual level regards the mechanism of shielding, I do not for a moment doubt the wisdom of years of practical experience so this is more about my wish to conceptually understand what's actually going on.

Suppose an insulator acted only as a physical (ion) barrier thereby increasing current path length and hence resistance to an area with an intense field, say a point or edge. Then if the insulator does not affect the field only the response of ions to that field then there will still be an area of intense field both sides of the insulator and in particular the region between the sharp point or edge and the shield may become ion depleted and hence cause burning unless we add agitation. However if we add agitation have we not simply undone the whole point of adding a shield - surely we have on one hand increased path length and then on the other hand offered ions a neat short cut by means of bulk import at speeds well in excess of ion migration velocity. Seen from this angle it starts to look a bit self contradictory.

https://en.wikipedia.org/wiki/Electrical_mobility

The key points I am making are that current or ion migration is a response to an electric field, stopping or throttling down ion migration does not imply the field itself is blocked - what happens in the region between the insulator and the sharp edge? this is the area of high field strength, our scenario dictates we have merely partially isolated it - we have not decreased the field strength and so can expect local effects including burning and ion depletion. If we do not encounter some ion depletion then the shield is not working correctly as we have a region between the sharp edge and insulator that still has a high field strength yet we have throttled back ion migration into that area.

Here I think its very important again to distinguish between a field and the current that results from it, in this scenario the insulator is NOT affecting field strength merely the response of ions to that field, we have to think first of field and then of current.

Again - I am not flying in the face of years of practical experience, I am interested in whats actually going on here and we can only do that by asking questions.

Coming from a Physics background I should know all about insulators in electric fields but I must admit I am rusty so I took a quick refresher "course" see this link.

http://physics.info/dielectrics/

The principle of interest here is the way that a dielectric responds to a field between two conductor plates by polarisation, setting up an opposing charge potential that decreases the field strength set up by the charged plates.

Now if the mechanism of the insulator is both to increase path length and so increase path resistance but also to reduce the field strength then there is potentially a double effect here and the reduction of field strength may go some way to answer the issues that I raised above.

To recap if we do not reduce field strength with insulators but merely increase path length then are we not laying ourselves open to a region of now isolated high field strength and so ion depletion which then warrants agitation which undoes the depletion but also arguably undoes the very benefit of increased path length? We seem to have hit a rather circular concept?

If the dielectric polarisation view is to be believed then we have an explanation that breaks the circular argument I suggest above because now the insulator is seen as being responsible not only for an increase in current path length but also as a reducer of high field strengths so we reduce the potential for ion depletion as the core problem of high field strength is reduced - the problem is solved at root cause rather than being side stepped.

If there is sense in this view then it would seem that good shields should be made from plastics with a high dielectric constant like PVC and indeed the same materials used in capacitors would make good candidates for shielding subject to other criteria of course.

It may also be the case that the best shapes for shields might be determined by consideration of symmetry so a rough approximation.

For instance I would guess that the best shape to diminish field strength between two conductors, one a hemisphere and the other a sharp point on the axis of that hemisphere would actually be a smaller hemisphere.

The ideal shape to reduce charge to the rim of a cylinder might be a half bagel ( sliced torus ) kind of shape, of course the shape and positioning of the anode will affect the field so these are only rough approximations, for a more detailed analysis we might consider making a shield shaped like a surface of equi-potential between the cathode and anode surfaces.

Sorry if this seems like a load of navel gazing to some -- it's not for me, I tend to like to proceed into areas like this with a conceptual idea of what's really going on so I hope those who feel differently will not shoot me down with arguments of experience or practicality and see that some folks like to know what's going on under the hood.

Jon Light - Saltum Denmark

November 2015

A. Hi Jon. You may be viewing it correctly, but I don't think so. I think the ions are not etherially responding to your 'field strength' argument, but are suffering their way along resistive paths.

As a thought experiment, picture it this way: imagine that ions must travel along copper wires like electrons do. Imagine that you have therefore installed 10,000 small copper wires evenly spaced over the surface of the anode, which then run to the cathode where they are again evenly distributed. Because the wires to the rim area are much shorter and therefore lower in resistance, you get more current flow to the rim, and heavier plating and/or 'burning'.

Put the shields in place and re-hook 10,000 wires. No drilling holes in the shields though, so now the wires to the rim area must go around the shield and are longer than the other wires rather than shorter. Their resistance is higher, so less plating takes place. If you find this thought experiment repugnant, replace the copper wires with small straws full of conductive solution :-)

Although the ions take conductive fluid paths rather than copper paths, those paths have resistance, and I think this is the correct way to picture it, and the way shields actually work.

Hi Jon
I admire your persistence but you have opened a subject that cannot be answered in a few words.

I will try. Conduction in solids is by transfer between the outer shell electrons. In electrolyte solutions current is carried by ions. The number of these depends on the concentration and degree of dissociation, temperature, viscosity etc.. In addition cations are not the simple ions shown in elementary texts but are solvated i.e. associated with one or more water molecules. This affects their mobility and their relative mobility is measured by the transport number. You will find a much more complete explanation with (many pages of maths) in any standard textbook of Physical Chemistry (e.g A.J. Mee)
For the application of this to electroplating I would suggest Technology of Electrodeposition A.T.Vagramyan, Draper, 1961. At a quick count he devotes 76 pages to the subject - which is why I shall stop here.

Please do not rely on wikipedia for a comprehensive explanation. It is often a good place to start but barely scratches the surface.

Geoff SmithHampshire,
England

November 19, 2015

Q. Hi Ted

Thanks for the reply.

I think your picture of thousands of copper wires of different lengths is the correct one to describe general ion flow in a non-agitated situation with no shielding. A simple hull cell test would show different path lengths and different deposition rates.

So for me the issue here revolves around what I see as possible inconsistencies in the arguments surrounding agitation and shielding.

Can we just call agitation bulk ion movement?

Two propositions which I have taken to be true.

1. Shielding can be used with arbitrary rates of bulk ion movement with no downside.
2. Shielding works by physically blocking ion migration along a favoured path to the benefit of a less favoured path - another analogy would be what happens to traffic distribution when an arterial road is closed for road works.

I would like to re-emphasise that electric fields can exist in a vacuum, as soon as we loop two plates to a battery we have a field, we don't need ions or transport mechanisms to have a field, current is how charge reacts to a field it is not the field itself, resistance is simply what obstacles we place in the way of charged particles reacting to a field, resistance does not affect the field.

Also that there is no distinction in Physics between what people call Electrostatics and regular electrical circuits, the differences and distinctions only arise because of the materials people place in an electric field between charged bodies and the potential difference (voltage) between those bodies; if you place a conductor with free electrons between two moderately charged plates (few hundred volts you will have conventional domestic electricity) but a few thousand volts between the same plates and a different medium between the plates - say a dielectric and you have a capacitor. The field is simply about things like charge, shape and location, the rest is about how matter interacts with that field.

So we have to mentally separate out the electric field and the substances in between the plates.

That is not to say that the substances you place between the plates do not alter the field, the dielectric capacitor is such an example but the important thing is that it reduces field strength by becoming polarised and so producing an internal electric field that opposes the external one. You alter fields with other fields.

This is an important point, you cannot block an electric field in some neutral inert way, you block it by introducing an opposing field or by introducing something like a dielectric that will create that field for you (back to capacitors).

So the picture of charge migration is of ions or other small charged particles moving under the influence of an electric field and they CAN be blocked by a physical barrier BUT the electric field can only be blocked if we add something that produces an opposing electric field to the main one and again this is related to my point since the latter alludes to shields that work by actually countering the main electric field and not merely by physical blocking of ion migration although the two are not mutually exclusive.

Returning to the shield, sure the mechanism could be pure physical blocking of ion migration forcing longer paths but if this were the case then I would argue that bulk ion movement (agitation) would undo our good work. Ions move very slowly compared with electrons in wires and much slower than the speeds involved in bulk migration (agitation). Since in this scenario the shield has done nothing to alter the electric field then there is still a high strength field on the "dark side" of the shield - as soon as we inject a high concentration of fresh ions into that region then they can act just as before - they don't know their history, they are transported suddenly into an intense field and will behave accordingly - how they got there becomes irrelevant.

So here there is for me a conflict between the explanation of shields as simple path blockers and the effects of agitation, I would expect sufficiently strong agitation or bulk ion movement to pretty much undermine the desired effect of shielding - bulk transport neatly side-steps the burden of path length as ions can be transported to any local area of high strength field very rapidly - they would get depleted by the high rate of deposition caused by the high field but sufficient agitation ensures they are replaced as fast as they are depleted and path lengths become irrelevant.

That's why I started to wonder whether the dielectric nature of insulators might offer an explanation where the shield is actually altering the local field - the only way it can do that as far as I am aware is by becoming polarised and generating an opposing (canceling) field.

I am not suggesting that a shield does not also work by physical blocking but I am saying that if it worked exclusively by such blocking with no effect on field strength then I would expect to hear that the benefits of shields are lost as soon as sufficiently high rates of bulk movement or agitation are introduced.

If someone on the forum says - "yes, that's right you have to keep agitation down to sensible limits when using shields otherwise the benefit of the shield is completely lost" then I would see no contradiction and no need for anything more complex than the simple blocking explanation. I would expect to find out that polarisation of the shield reduced field strength but perhaps not enough for it to be a particularly important factor in the shielding mechanism.

The "evidence" suggesting another mechanism is at play for me boils down to my understanding that agitation is not counter productive to shielding.

Jon Light - Saltum Denmark

November 19, 2015 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello people.
I'm facing an issue while electroplating Ag on Cu.
Not able to get constant thickness on job.

For ex: job pieces at center of the jig are OK with the thickness whereas, the pieces placed on the start and end of the jig have more thickness in spite of correct contents and current as per area.
Kindly help me with the solution/ answer for the same.
Thank you.

Nikhil SPlating Shop Employee - Nasik, Maharashtra, India

November 2015

A. Hi Nikhil. We appended your inquiry to a thread which talks about the many different approaches to resolving this problem. But the easiest and most obvious way to improve your situation sounds like removing the anodes at the ends of the tank.

A. Hi Rajkamal. Per Faraday's Law, the thickness at any spot is directly proportional to the current flow to that part (this is a slight exaggeration as there are some more obscure relationships between thickness and other partameters, but it's almost surely the problem here). To get even plating thickness to different spots you'll need to insure even current flow to the different spots. One fairly common problem is anode rods being too long, so you get a lot of current flow to the bottom of the part. Good luck and feel free to send sketches.

Q. I Want to Reduce the Nickel Percentage from Present of around 5 - 6 Microns to 1 - 2 microns. Please I Need help.

Pintu Shah- Jamnagar, Gujarat, India

September 2016

A. Hi Pintu. Your posting is a bit too short to be understood. Are you saying that your present nickel plating thickness is 5-6 microns and you want to reduce the thickness to about 1-2 microns, or are you saying that the thickness varies over the part by 5-6 microns and you would like to reduce the variation to 1-2 microns?

Reducing the thickness to 1-2 microns would be very easy; simply cut the plating time or the plating current proportionally. But there are very few applications where 1-2 microns of nickel plating would be satisfactory. Perhaps for certain diffusion coatings or strikes, I suppose, but not as an actual plating layer. 5 microns is required for even "mild" environmental conditions (indoor applications). What are the parts? What is the application?

Reducing the variation in plating thickness is difficult. We would need a lot of info about the substrate material and geometry, and the target plating thickness. Good luck!

I realize electrolytic nickel on aluminum terminals. After measuring the thickness of nickel, in some areas, we lose up to 50% of thickness (area marked in red).

Terminals are screwed on a rack that holds about forty terminals. The cathode is placed parallel to the tongue of the terminal, at mid-height of the rack and all the terminals.
I have read some posts and I think geometry favors higher or lower current density. Maybe the distance terminal/cathode can also be incriminated.
So, this is my question: how I can greatly reduce this variation in thickness?

Regards,

Jeremy FARGE - Pompadour, Limousin, FRANCE

November 4, 2016

A. Hello Jeremy,
I would first check the calibration on your rectifier. Next, contact points between anode and cathode buss bars, and last for any polarization in your anode material supply. Although it is not uncommon to have a fluctuation in thickness from high to low current density areas, or an area where metal must travel further than the shortest path to the work, 50% less seems a bit high for the part in your image. If once you have insured all currents and anodes are preforming correctly you are still left with an an unacceptable difference in thickness, it's time to consider auxiliary anodes. An auxiliary anode is a supplementary anode that alters the current distribution in electroplating to give a more uniform plating thickness. A small piece of anode material placed in close to the area you are having trouble with connected to the anode current supply.
If it is still not as uniform as you would like you could move over to electroless Ni. Electroless Ni solutions are touted for their uniformity in thickness compared to any electroplating solution. Good luck.

A. Hello Jeremy, Chance had some good suggestions and I just wanted to add a few more. I experienced a similar problem a few years ago and was given the task to reduce thickness variation in our Ni plating tank. First I had to bring the anode to cathode ratio down to 2:1. We were at about 5:1. Then I lowered the current density to 20 ASF, we were at 28. Higher current densities will favor the high CD areas on the part. Lower CD's will help you deposit more metal in the LCD areas. You will have to increase dwell time accordingly to meet your thickness specs. Hope this helps.

Mark BakerProcess Engineering - Phoenix, Arizona USA

Ideal Anode to cathode distance in Nickel plating

My problem is Nickel plating in some small holes which is not desired. Is anode cathode distance playing a role? Of course higher distance will result in loss of electricity (high voltage) is there any standard desired distance? Too close anodes of course creates roughness. Any maximum voltage? Current density desired as high as possible.

Kaushik Magiawalaconsultant - Adalaj, India

July 2017

A. Hi Kaushik. Sorry but I do not understand the main question regarding you getting unwanted plating in holes. Unwanted plating is usually stopped by masking; but since I am misunderstanding you, a picture with labels showing the problem would probably help.

My career was mainly as a plating equipment engineer, designing, starting up, and troubleshooting plating lines. So I know very little about the deeper academic end of electrochemistry, but feel that most questions of anode-to-cathode spacing revolve around the simple practicalities: You don't want the anode and cathode to ever accidentally touch, and you don't want them so close that the plating on the close spots burns; but you want them reasonably close because solution resistance is terribly costly. Greater spacing requires higher voltage rectifiers, consumes more electrical power, requires more cooling, needs bigger tanks with greater capital cost, requires more fume ventilation and more make-up air, etc. This is, to me, the 'desired standard distance'. Again, for a deeper understanding you'll need an answer from someone with different background & experience.

But I do not agree that close spacing "creates roughness". Roughness, as most people use the term, implies the deposition of particulates ... whereas if the anodes are well bagged and you have efficient filtration, roughness should not be a problem. Too much current causes burning, which is rough, and this may be what you are referring to. I think you might look into the 'shields' discussed on this page as a way to minimize that burning, and to discourage plating in the areas where you don't want it. Best of luck.

To have good leveling a longer distance between anode and cattode is important; if it too close the current will go a shorter way. If there are holes in the part you should use inductive/auxiliary anodes.

Regards

Anders Sundman3rd Generation in Plating
Consultant - Arvika, Sweden

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